I don't think we can do anything but speculate anecdotally; the only way to know why you never heard about it would be to check with your former teachers or textbooks. My assumption here is that you're curious why it might be missing from a general education class like World History.
Without knowing much detail about when/where/who the OP took history, I can only hasard a guess that it was left out as it is much more difficult to encapsulate than the Atlantic Slave Trade-- and its effects on "Western Society"-- are much less keenly felt than the Atlantic Slave Trade. That is, the Atlantic Slave Trade was carried out for about 400 years, beginning as as trickle and gradually reaching a flood of people being brought across the ocean in chains. The North African/Arab slave trade was carried on for near 1500 years. Though each of these economic systems are estimated to have involved about the same number of enslaved persons; one was diffused across a much larger time period and is therefor far more difficult to encapsulate in one lesson or lecture or another.
At the two institutions I have taught at the Arabian slave trade came up in four different places-- but it was never the focus of any unit and so its inclusion in one or all of these places was usually up to the professor's lecture style and whether or not the student read the textbook. It came up in the World History Pre-1500 CE class as part of a series on the Trans-Sahara Trade Network and Western African Kingdoms/Empires and in European Oversees Exploration (specifically Portuguese sailing down Africa and Atlantic sugar islands). It is also mentioned in the World History 1500-Present class during the Trans-Atlantic Slave Trade as a brief comparison in terms of experiences of enslaved individuals and then somewhat in passing during the Gunpowder Empires/Islamic Empire unit (Ottomans, Mughals, Safavids). I additionally sometimes mention Mediterranean slavery depending on the semester schedule-- as fear over enslavement by Muslims and the enslavement of Muslims on galleys-- became an important part of French oversees diplomacy between 1400-1600. Vast amounts of money were spent on manumission of Europeans captured (at least for those whose families could pay) and a great deal of legalistic (or quasi-legal) wrangling was done to explain why it was okay to have slaves on French galley ships.
(if you interested in that topic I would definitely recommend : Weiss, Gillian. Captives and Corsairs: France and Slavery in the Early Modern Mediterranean. Stanford: Stanford University Press, 2011.)
Not every institution does things the same way though, and not every Professor/teacher presents material in the same way. I am sure others just skip the matter entirely as not relevant to the point they are trying to make-- when you only have 45-60 minutes to cover hundreds of years of history, some things get cut, especially if they might be in the textbook-- and certainly if the topic cannot be easily encapsulated. Some textbooks, especially those geared more towards a Western Civ focus might leave out the topic entirely as not relavent to the march of Western Civ. Atlantic Slavery has an obvious connection to Europe and the Americas, but Arabian slavery is perhaps a bit more oblique (at least for a General Ed class).